1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly, to an optical pickup device configured to detect a stable error signal by reducing offsets even when the focusing means such as an objective lens is shaken, there is deviation in a photodetector and/or there is a change in the temperature.
2. Description of the Related Art
In a high-capacity recording and/or reproducing optical pickup device, focus and/or track error signals must be detected in order to perform a stable servo function. Typically, an optical pickup device includes a light source, an objective lens for focusing a beam emitted from the light source onto the recording plane of a disk, and a signal detection unit for detecting an information signal and an error signal from light reflected from the disk and passed through the objective lens.
In the case of an optical pickup device configured to detect a focus error signal by an astigmatic method, the signal detection unit has an optical arrangement as shown in FIG. 1. Referring to FIG. 1, beams reflected from a disk (not shown) are focused by a sensing lens 2 via an objective lens (not shown) and are received in a photodetector 6 via an astigmatic lens 4 for producing astigmatism. The photodetector 6 has four light receiving regions A, B, C and D arranged in a 2xc3x972 matrix.
When the signals detected from the respective light receiving regions A, B, C and D of the photodetector 6 are referred to as the same characters, respectively, a radio frequency signal RFS is obtained by summing the signals detected from the light receiving regions A, B, C and D, as represented by formula (1):
RFS=(A+B+C+D)xe2x80x83xe2x80x83(1)
A focus error signal FES is obtained by summing the signals detected from the diagonally opposite light receiving regions, i.e., A+C and B+D, and then obtaining a difference between the sums, as represented by formula (2):
xe2x80x83FES=(A+C)xe2x88x92(B+D)xe2x80x83xe2x80x83(2)
A track error signal TES based on a push-pull method is obtained by summing the signals detected from the light receiving regions parallel to the track direction, i.e., A+D and B+C, and then obtaining a difference between the sums.
TES=(A+D)xe2x88x92(B+C)xe2x80x83xe2x80x83(3)
In the case of recording/reproducing on/from a high-density recording/reproducing disk, e.g., a DVD-RAM disk, which adopts a land/groove recording type, using an optical pickup device having the conventional signal detection unit, even at an on-focus state, there is severe cross talk in a focus error signal based on an astigmatic method, due to the effect by neighboring tracks. Thus, the optimal positions of focuses of land/groove differ.
Further, since the conventional signal detection unit has a small-sized light spot formed on a photodetector, a focus error signal and a track error signal are sensitive to deviation of the photodetector. Accordingly, focus offset and track offset, in which the focus error signal and the track error signal become a value other than zero at an on-focus and on-track positions, may occur.
Also, the wavelengths of light emitted from a light source are changed according to changes in the temperature. Thus, if chromatic aberration occurs to focusing means, for example, an optical element such as an objective lens (In most optical elements, an increase in the wavelength reduces the refractive index.), focus offset, in which the focus error signal have a value other than zero, occurs even at an on-focus state.
Also, if an objective lens is shifted from its original place by a seek or a disk eccentricity, beams are shifted on a photodetector. Thus, an offset occurs to a push-pull signal.
To solve the above problems, it is an object of the present invention to provide an optical pickup device free from offset in a track error signal by shaking of focusing means such as an objective lens, by which high-density recording/reproduction is allowed, and the optical pickup device free from offset in a focus error signal by deviation of a photodetector and/or a change in the temperature, by which a stable error signal can be detected.
To achieve the above object, there is provided an optical pickup device including: a first light source for generating and emitting light; first light path changing means for changing a traveling path of incident light; focusing means for focusing the incident light from the light path changing means onto a recording medium; and a first signal detection unit having a hologram element for diffracting incident light after being reflected from the recording medium, a photodetector for receiving the light diffracted by the hologram element and converting the same into an electrical signal and a signal operation unit for generating a focus error signal and/or a track error signal from a detection signal of the photodetector, the signal detection unit for receiving the light reflected from the recording medium and then passed through the focusing means and the first light path changing means, wherein the hologram element includes first and second pattern portions arranged to be spaced apart a predetermined distance from each other, for diffracting and focusing incident light, and third and fourth pattern portions arranged close to the first and second pattern portions, respectively, for diffracting incident light in different directions, the first and second pattern portions being provided such that xc2x11st-order diffracted beams are focused on first and second focuses, respectively, and wherein the photodetector includes first and second light receiving units positioned between the first and second focuses in an on-focus state, for receiving the light diffracted and focused by the first and second pattern portions, and third and fourth light receiving units for receiving the light diffracted by the third and fourth pattern portions.
Here, the first pattern portion preferably includes first and second pattern regions, the second pattern portion includes third and fourth pattern regions facing the first and second pattern regions, respectively, one diffracted beam among xc2x11st-order diffracted beams diffracted by the first and fourth pattern regions is focused on the first focus and the other diffracted beam is focused on the second focus, and one diffracted beam among xc2x11st-order diffracted beams diffracted by the second and third pattern regions is focused on the second focus and the other diffracted beam is focused on the first focus.
In this case, the first light receiving unit may include first and second sectional light receiving regions each of which forms a bisectional light receiving region for receiving one diffracted beam among the xc2x11st-order diffracted beams diffracted by the first pattern region, and third and fourth sectional light receiving regions each of which forms a bisectional light receiving region for receiving one diffracted beam among the xc2x11st-order diffracted beams diffracted by the second pattern region; and the second light receiving unit includes fifth and sixth sectional light receiving regions each of which forms a bisectional light receiving region for receiving one diffracted beam among the xc2x11st-order diffracted beams diffracted by the third pattern region, and seventh and eighth sectional light receiving regions each of which forms a bisectional light receiving region for receiving one diffracted beam among the xc2x11st-order diffracted beams diffracted by the fourth pattern region.
Preferably, the bisectional light receiving regions are sectioned substantially parallel to each other, and the second and eighth sectional light receiving regions and fifth and third sectional light receiving regions are disposed inward with respect to the first and seventh sectional light receiving regions and the sixth and fourth sectional light receiving regions, respectively.
The signal operation unit detects a focus error signal by obtaining a difference between the sum signal of detection signals of the first, third, fifth and seventh sectional light receiving regions and the sum signal of detection signals of the second, fourth, sixth and eighth sectional light receiving regions.
Also, the first light receiving unit may further include first and second single light receiving regions for receiving another diffracted beams diffracted by the first and second pattern regions, and the second light receiving unit may further include third and fourth single light receiving regions for receiving another diffracted beams diffracted by the third and fourth pattern regions.
The third and fourth pattern portions are provided between and/or outside the first and second pattern portions.
Here, the space between the first and second pattern portions and the widths of the first and second pattern portions are optimized so as to suppress offsets of a track error signal against the movement of focusing means of the optical pickup device.
In order to detect a push-pull signal without a direct-current (DC) component offset, the signal operation unit preferably includes a first differential unit for obtaining the difference between the detection signals of the first and second light receiving units, a second differential unit for obtaining the difference between the detection signals of the third and fourth light receiving units, first and second amplifiers for amplifying output signals of the first and second differential units into predetermined gain levels, and a third differential unit for obtaining the difference between the signals input from the first and second amplifiers, wherein the gains of the first and second amplifiers satisfy the condition of the DC component offset in the differential signal of the third differential unit being removed.